The present invention relates generally to highly effective thermal insulation, and more specifically to methods for fabricating a micro heat barrier comprising a highly reflective membrane supported by micron-scale microsupports fabricated using semiconductor and MEMS processing techniques. The present invention also relates to processes for fabricating micro heat barriers.
A need exists in a variety of industrial, scientific, and military applications for a highly efficient, lightweight thermal insulation. In particular, microelectronic and micro-electro-mechanical-systems (MEMS) devices need very thin (micron-scale) advanced thermal insulators to enable efficient thermal management schemes. An advanced Micro Heat Barrier (MHB) can be used for a variety of thermal management applications, such as:
Controlling parasitic heat flow in a variety of integrated circuit (IC) devices, surface emitting lasers (LED, VSCEL) for communications and lighting, micro-cryogenics, micro-heaters for micro chemlab on-a-chip, etc.,
Enabling micro power conversion devices for MEMS, micro sensor, or micro chemlab applications, including self-powered micro thermionic converters, micro thermovoltaics, micro thermionics, and micro thermoelectric devices,
Thermal control of micro sensors, and
Thermal control for microbiological cell culture applications.
For applications like self-powered microthermionic converters made using IC, MEMS, and surface micromachine techniques, temperature gradients of more than 1100 C. must be maintained over very short distances (e.g., less than 150 microns). Unfortunately, the very short thermal conduction path severely limits the ability of conventional bulk thermal insulators, such as silicon, silicon dioxide, or silicon nitride, to provide effective thermal isolation using. Hence, advanced thermal barriers are needed.
Effective thermal insulators use an insulating material having low thermal conductivity (e.g., ceramic, plastic, glass), combined with a vacuum gap and/or reflective internal surfaces (e.g., Thermos(trademark) bottle). While a vacuum gap eliminates thermal conduction over the region not in contact, some type of mechanical support is needed to maintain the gap. As a consequence, heat loss can occur by conduction through the supports. On the micro scale even modest contact areas of more conventional insulators results in unacceptably high thermal conduction heat losses. Furthermore, at high temperatures heat loss across the gap can result from thermal radiation. Radiative heat transfer (Infrared Radiation, IR) is minimized by using highly reflective (i.e., non-absorbing) surfaces (e.g., polished metal surfaces).
Highly efficient thermal insulators have been made by alternating layers of reflective material (aluminum foil, copper foil, or aluminized Mylar(trademark)) separated by low thermal conductivity spacers (e.g., fibrous material),; or by crinkling the reflective material to allow contact at only a few points. Operation in a vacuum further improves the thermal insulation by eliminating conduction and convection in the gas. This type of multilayer insulation is commonly used for cyrogenic insulation. Such a multilayer insulator can have an apparent thermal conductivity as low as 10xe2x88x925 W/m-K, which is approximately 100,000 times less conductive than quartz.
For high temperature heat engines, solar Stirling engines, and radioisotope or thermionic space power sources, materials with high melting points must be used for the multilayered insulation. One commercially available example is the MULTI-FOIL(trademark) insulation developed by Thermo Electron Corporation. A typical MULTI-FOIL(trademark) uses 60-80 stacked layers of thin metal foils (e.g., 12.5 microns thick) of reflective metals (e.g., niobium, molybdenum, zirconia-coated aluminum) separated by small particles of low conductivity oxides (e.g., zirconium oxide). The fabrication process (and corresponding large dimensions) developed for manufacturing multilayer insulations, such as MULTI-FOIL(trademark) insulation, makes this technology unsuitable for use in MEMS and microthermionic converters. Also, the thermal emissivity of metal foils ranges are relatively high (0.3-0.4) due to the native raw oxide present on the foils.
In U.S. Pat. No. 6,197,180 Kelly teaches a process for fabricating high aspect ratio microstructures by electroplating nickel using the LIGA process to form a heat shield comprising a forest of nickel xe2x80x9cumbrellasxe2x80x9d connected to the substrate by nickel microposts, where the microposts have a height of 100-1000 microns. The use of electroplated nickel microposts, however, limits the ability to achieve highly effective thermal insulation because of nickel""s high thermal conductivity. Also, the height of the microposts (100-1000 microns) limits their use in MEMS or other microelectronic devices (which require thermal insulation barriers with a thickness on the order of 10 microns, not 100-1000 microns).
A need exists, therefore, for a highly effective, micron-scale micro heat barrier structure and a process for fabricating, that combines the advantages of multilayer vacuum thermal insulation with the micron-scale fabrication techniques of semiconductor, micromachine, MEMS, or microthermionic devices, having an apparent thermal conductivity much lower than existing multifoil insulation, and that can be directly integrated into the fabrication process of the semiconductor, micromachine, MEMS, or microthermionic device.
Against this background, the present invention was developed.
The present invention relates to methods for fabricating a highly effective, micron-scale micro heat barrier structure and process for manufacturing a micro heat barrier based on semiconductor and/or MEMS fabrication techniques. The micro heat barrier has an array of non-metallic, freestanding microsupports with a height less than 100 microns, attached to a substrate. An infrared reflective membrane (e.g., 1 micron gold) can be supported by the array of microsupports to provide radiation shielding. The micro heat barrier can be evacuated to eliminate gas phase heat conduction and convection. Semi-isotropic, reactive ion plasma etching can be used to create a microspike having a cusp-like shape with a sharp, pointed tip ( less than 0.1 micron), to minimize the tip""s contact area. A heat source can be placed directly on the microspikes. The micro heat barrier can have an apparent thermal conductivity in the range of 10xe2x88x926 to10xe2x88x927 W/m-K. Multiple layers of reflective membranes can be used to increase thermal resistance.